Visitor From Another Timeline
by inukagome15
Summary: There was no resetting, no turning back the clock and erasing everything that had happened… Everything was good for once. So why did his luck have to change for the worse?


**For the logistics of the plot, I'm claiming wibbly-wobbly timelines and leaving it at that. This was a prompt from an anon on my tumblr who asked for Sans having to use his abilities in front of the others. I had to think about this for a little, since Sans literally doesn't use his abilities until there's nothing left and he's the last line of defense. kurohi91 was a very big help for brainstorming. Thanks!**

 **Fonts and everything are in their proper format on AO3. FF insists on being that crotchety old grandparent stuck in the 90s, so you're all settling for basic fonts here...**

 **Visitor From Another Timeline : **There was no resetting, no turning back the clock and erasing everything that had happened… Everything was good for once.

So why did his luck have to change for the worse?

* * *

 **Visitor From Another Timeline**

* * *

This wasn't the first time they had come to the surface. But it was the first time that they had _stayed_. For a lot longer than what Sans remembered last time.

There was no resetting, no turning back the clock and erasing everything that had happened… No, everything was _good_.

It was more than Sans could have expected or hoped.

Or not hoped, really. He hadn't hoped for anything in a long time.

And it was so easy to forget that under the stars of the surface. He could distantly remember seeing them in another time, but seeing them again in the present was so much better. Much better than the "stars" they had underground.

Sans' telescope actually got a workout now, especially out in the country where they were now.

Papyrus didn't sleep, but he also didn't seem to mind that Sans went out in the middle of the night to look at the stars. In fact, he seemed downright gleeful over this because it meant that Sans wasn't "NAPPING HIMSELF INTO AN EARLY GRAVE." Frisk and Undyne hadn't yet managed to convince Papyrus that this was called sleeping and something perfectly healthy.

Papyrus had seemed rather horrified to learn that Frisk needed more than seven hours of sleep at night to function properly. Sans still liked to remember when Toriel had kicked Papyrus out after several days of Papyrus trying to keep Frisk up.

There had been quite a lot of fire magic involved…

On the other hand, Sans still napped, but it wasn't as bad as it had been before…

Tucking his right hand back into his jacket pocket, Sans returned his attention to the telescope, fiddling with the controls and bringing Mars into sharper focus. Mars was in a good position tonight, even with the moon's light blotting out a lot of the stars.

He almost didn't hear the footsteps behind him, the sound of shoes dragging themselves across the grass.

Sans half-turned his head, mildly surprised to see Frisk approaching. They were supposed to be asleep at this time, not sneaking out to see Sans under the starry night sky.

Something glinted in their hand, silvery metal catching the light of the moon and stars. Something that Sans hadn't seen since…another lifetime.

They were shambling across the grass, limbs jerking strangely and not quite as smooth as Frisk usually was. And their body…was covered in dust.

 _Monster dust._

"kiddo…" Sans couldn't keep the wariness out of his voice, nor did he try. He did step away from his telescope, putting a foot more space between the two of them. "what's going on?"

Frisk – whoever this was – moved closer, hair hiding their face from view. But Sans could very well picture what was on it, having seen it more times than he could count through reset after reset, timeline after timeline…

He dreaded seeing it now – didn't _understand_ why this was happening.

They were all fine. Things had turned out _great_. They were all on the surface and everyone was _happy_.

So why was Frisk doing this?

"frisk, buddy…" Sans' shoulders tightened as Frisk inched closer, that damned knife glinting in their hands despite the thick layer of dust coating the blade.

Frisk lifted their head, the knife coming up with the movement as well. They looked straight at Sans, their eyes narrowing when they saw him, mouth twisting into a disgusted grimace. There was something like a glint of red in their eyes, but it could just have been Sans seeing things in the dark.

But their LV and EXP… Sans hadn't seen that with this Frisk in this timeline, and there was no way they could've reached that level in the few hours since he'd last seen them.

"that look on your face…" The words came to him as if in a dream, ones that he had said before. "guess i'm pretty good at my job, huh?"

Frisk drew the knife across their throat, not looking away from Sans.

"how many times does that make for you?" Sans risked a glance behind Frisk, wanting to know (yet _not_ wanting to know). Was Toriel all right? Undyne? _Papyrus_? Please let Papyrus be all right. Sans couldn't take another timeline of his brother being dead. "more than ten? twenty?" His eyes went dark. " **Do you want another round?** "

Predictably, Frisk didn't react beyond their head tilting slightly, assessing Sans closely.

Without warning, they slashed out with the knife, lunging forwards towards Sans.

Sans dodged, sidestepping the attack with a rush of dread. "you think i'm just going to stand there and take it?" He felt like he was reading off a script, one he'd covered countless times before, the words and movements utterly familiar.

Sans didn't want to fight – he never did, really. It was too much effort, and his magic was too destructive if he gave it free reign. But he wasn't being given a choice here.

"all right, kid…" Sans hunched his shoulders, briefly closing his eyes. "don't say i didn't warn ya."

He wasn't too surprised when the kid dodged his first attack, slipping through the gaps between his attacks. He sidestepped the kid's next knife slash, retaliating with a deluge of bones that had them jumping back to avoid being skewered.

It put more space between them, giving Sans much needed breathing room. He closed an eye, latching onto the kid's soul before they could react, turning it blue. They slammed into the ground, weighted down by their sins.

"you can call it quits now, kid," Sans said, right eye dark.

The kid didn't say anything beyond a wordless snarl, pushing up to their feet. They staggered slightly before steadying, furious eyes meeting Sans'.

"welp…" Sans closed both eyes, shrugging once. "i tried."

Before the kid could react, Sans' left eye flared yellow and blue, the blasters materializing around them, crackling with energy. They fired a split-second later, gouging deep lines into the ground, spraying dirt and grass everywhere.

The attacks hit the kid, but not dead on. Still, it was enough to begin dropping the kid's HP, and by the time the energy blasts dissipated, Sans could see them stumbling unsteadily. Their fingers were clenched around the knife, the blade shaking.

"it didn't have to be this way," Sans said, subdued. "we had a good thing going, you know? so why? why are you doing _this_?"

The kid didn't respond, lunging forwards to attack Sans.

He sidestepped the knife slash, sending bones flying in their direction, but then had to hastily sidestep another attack and another before he could react. The third nicked his jacket, and Sans stepped sideways, reappearing a dozen feet behind the kid, anxiety skittering down his spine.

He didn't like remembering his own death, but he was pretty sure it had happened at some point.

 _"papyrus, do you want anything?"_

"not this time, buddy," Sans said, flicking the kid sideways with a twist of his wrist. "and never again."

There was a rhythm to fighting, something one fell into with the ease of practice and time. It meant Sans blocked out most everything but the dangerous figure of the kid, dodging their attacks and retaliating in kind. They dodged some of his attacks, but he scored more hits against them than they did against him.

But if this went on too much longer, he'd run out of stamina. Ending this quickly was a better idea than dragging it out for however much longer the kid could go.

And Sans knew the kid could go on for a lot longer with the right items.

"y'know…" Sans hemmed the kid in with a horde of blue bones, forcing them to stand still. "this is why i really hate making promises."

Things had been so _good_. They had all been genuinely happy. Even Sans had tentatively really started to enjoy himself, trying not to let the fear of another reset drag him down.

Frisk was a friend, one who laughed at Sans' puns and adored Papyrus as much as Sans did. It was easy to forget what the power of their determination could do when confronted with such an innocent child.

But _this_ kid…wasn't innocent. Sans _remembered_ what they had done, what he had been forced to do as a result.

And he hated that he had to do it again, because it meant that everything had gone _wrong_.

Left eye flickering between blue and yellow, Sans drew on his magic to materialize a large blaster. "don't come back." Don't make him do this again.

The blaster crackled with energy, glowing as it geared for the attack.

"SANS, STOP!"

Sans flinched slightly at the voice but didn't look away from the motionless kid, his focus on the attack.

Papyrus rushed in front of the kid, bristling defensively. "BROTHER, PLEASE!"

Papyrus wasn't moving. Shock and horror roiled through Sans, and it was with a jerk that he wrenched the blaster off target, firing its attack harmlessly into the sky. "p-papyrus…you—"

"FRISK IS OUR FRIEND," Papyrus insisted. "WHY ARE YOU DOING THIS?"

The blue attack that had been holding the damned kid immobile was gone, having dissipated during Sans' shock. Sans hadn't even noticed, and it wasn't until he saw them lurching forwards, knife heading _directly towards Papyrus_ —

Later, Sans wasn't even entirely sure what had happened, only that he'd _reacted_ , and there were blue bones piercing every single limb of the kid's body, stopping their attack before it could go through.

" **Not again.** " Sans trembled, left hand up defensively. " **Or you're dead.** "

"SANS?" Papyrus sounded faintly bewildered, turning his head to look at the kid. He flinched slightly upon seeing how they were standing, caught in the middle of a deadly attack.

Sans shuffled his feet, uncertain of how to explain this. As far as Papyrus knew, Frisk was still a friend. None of this made any sense to anyone who had no idea of alternate timelines and what had been happening.

"S-Sans…"

Sans instinctively turned, eye flaring blue, only to stop upon seeing…Frisk? They were standing there, shivering in the cool air, looking utterly lost and confused. Toriel was right behind them, eyes flickering between all of them as if she was dreaming and would wake up any moment.

This Frisk had no LV and no EXP. They were exactly as Sans had left them hours before. There was no dust on their body, and there wasn't a weapon in sight.

Turning back to the kid he was still holding immobile, Sans realized this was a little more complicated than Frisk suddenly losing it and deciding to go on a killing spree.

"so…you jumped timelines just to get at little ol' me?" Sans managed a wry laugh, shoulders shaking. "how'd you even manage that?"

They'd come at _him_ with a desperation borne of frustration and anger resulting from dozens of resets. Maybe they'd gotten past him _once_ by chance, but Sans had gotten _them_ a lot more than that.

"THERE ARE TWO FRISKS?" Papyrus sounded even more confused now, head rapidly twisting between the two identical children.

Sans let some of the tension seep out of him, exhaling through his teeth. He was aware that this looked pretty bad. The fight had left deep gouges cut into the earth, dirt and grass strewn about everywhere. The once pristine hilltop was utterly ravaged. Sans was the only one uninjured and the one currently holding the kid hostage at bone-point.

And he had little idea of how to explain this without sounding absolutely cuckoo.

"now there are," Sans said eventually, closing his eyes. "and…" He sighed, a heavy weight sinking down on him. His fingers twitched, the bones holding the _other_ Frisk immobile vibrating minutely with the action. He _wanted_ to finish the job but…not with them watching. The kid gave him a vengeful glare, evidently understanding exactly what Sans wanted to do and reciprocating the desire.

"i can't do this again." It was so pitifully easy to hear how exhausted he was that Sans cringed.

Papyrus moved closer – thankfully putting distance between him and that knife – and wrapping Sans in one of his patented hugs. "YOU DON'T HAVE TO, BROTHER. I AM HERE." Toriel cleared her throat meaningfully. "WE ARE ALL HERE," Papyrus amended.

Hesitantly, half his focus still on his magic, Sans wrapped his right arm around Papyrus, feeling the reassuring pulse of his brother's magic against his own. The other Frisk was covered in dust – that of Papyrus's and other monsters – but _this_ Papyrus was alive.

And Sans would do his damn best to make sure it _stayed_ that way.

* * *

 **After many explanations, Frisk confirming Sans' theories, they're left with a rather homicidal Chara in Frisk's body. It's Papyrus who volunteers to do the rehabilitation, believing there's still good in them. Sans refuses to leave his brother alone, which then leads to months of Sans eyeing Chara suspiciously and looming threateningly while Papyrus does his best.**

 **In the end they manage to dial down Chara's homicidal tendencies to a 5. No one lets them near any weapons.**

 **I hope this was a decent read. Please let me know what you thought!**

 **I am taking commissions if anyone's interested.**


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